Hello Cameron
΅When you use the default-information originate command alone, without the always keyword, OSPF will advertise a default gateway only if a default gateway exists in the routing table itself. If the always keyword is used, then it will advertise a default gateway regardless of whether or not a default gateway exists in the routing table.
To understand this more fully, take a look at this topology once again:
Let’s say that R2 has no default gateway configured in its routing table. Let’s say you issue the following command on R2 within the OSPF configuration mode:
default-information originate
R2 will not advertise a default route to R1.
If you use the following command:
default-information originate always
R2 will advertise a default route to R1. In other words, it will say to R1 “install in your routing table my next hop IP address (192.168.12.2) as the default gateway”. Thus, the default gateway that R1 will put in its routing table via OSPF is 192.168.12.2.
Does that make sense?
I hope this has been helpful!
Laz